1. Field of the Invention
The present invention relates to a power control device and an image forming apparatus employing the power control device.
2. Description of the Related Art
A power supply unit (circuit) supplies power from a capacitor or a charger to a fixing heater of an image forming apparatus. The power supply unit includes a semiconductor device (discharge transistor) that controls on (close)/off (open) of power and a relay as a safety device connected in series to the semiconductor device, both of which serve as a power control device (circuit).
The relay is controlled so that a contact thereof is turned on/off when the semiconductor device is off, and therefore, a current to the heater is not directly turned on/off at the contact of the relay. For this reason, a compact relay for a small current can be used as the safety device.
However, because the semiconductor device controls on/off of a large current, there may be a case where switching on/off occurs at the contact of the relay caused by a failure or a malfunction of the semiconductor device. As a result, a problem such as soldering of the contact of the relay may occur.
If the contact of the relay is soldered, a safety circuit does not operate normally, and a fixing unit is thereby heated to more than a predetermined temperature. In this case, a thermal fuse blows, and this prevents occurrence of an event such as emission of smoke or fire, however, there occur some problems that rollers of a fixing unit are degraded and replacement thereof is needed.
To solve the problems, for example, Japanese Patent Application Laid-open No. 2005-221773 discloses a method of detecting soldering of a contact of a relay or detecting either a malfunction of a relay or a short-circuit failure of a semiconductor device based on the following configuration. The configuration is such that a series circuit that includes the relay and the semiconductor device is connected to a fixing heater in an image forming apparatus to control power supply from a capacitor to the fixing heater. More specifically, the series circuit, formed of the relay and the semiconductor device that turns on/off power supply from the capacitor to the fixing heater, is provided with a unit of detecting two modes such as detecting a soldering failure of the contact of the relay and detecting an open-circuit failure of the contact of the relay or a short-circuit failure of the semiconductor device.
However, in the conventional technology disclosed in Japanese Patent Application Laid-open No. 2005-221773, it is quite impossible to find out whether the cause of the problem is the open-circuit failure of the contact of the relay or the short-circuit failure of the semiconductor device. Moreover, in the conventional technology disclosed in Japanese Patent Application Laid-open No. 2005-221773, it is impossible to detect the open-circuit failure of the semiconductor device.